Method for biological purification of effluents using biofilm supporting particles

ABSTRACT

The invention concerns a method for biological purification of effluents in mixed cultures using micro-organisms whereof part at least is fixed on solid supports. The invention is characterised in that said supports are activated so as to generate a turbulence in the reaction medium, the intensity of which is such that it reduces the production of biological sludge, the materials constituting said micro-organism supports being abraded and cleaned, while being retained in said reaction medium.

[0001] The present invention relates to a method for the biologicalpurification of wastewater employing a hybrid culture system usingbiofilm support particles. It also relates to a reactor or equipment forimplementing such a method.

[0002] It is known that the purification of municipal and industrialwastewater is often carried out biologically. In recent decades,processes have passed from those using free microorganism cultures toprocesses with cultures fixed on specific growth media for the purposeof reducing the size of purification plants.

[0003] Fixed cultures are employed either as a fixed bed, that is to saya microorganism growth medium is stationary in the reactor, or as amoving bed, in which case the support materials are small elements thatcan move freely in the zone of contact with the polluted water. Thesesupport elements may be moved either by mechanical stirring or byinjecting a liquid, or else by injecting a gas, especially air (this airpossibly being, for example, the air needed for the microorganisms tooperate when they are aerobic).

[0004] The creation and maintenance of a certain level of turbulence inthe reaction medium are useable for continuously abrading and cleaningthe support material for the microorganisms, this turbulence furthermoremaking it possible to limit the accumulation of fixed biological sludge.Such turbulence may be created, for example, by the intensity of the gasinjected into the medium. Reference may be made in this regard to EP-A-0549 443.

[0005] If it is desired to treat the pollution due to carbon and tonitrogen simultaneously, it is possible to find advantageous solutionsgiven that the materials serve as growth medium for a certain nitrifyingbiomass, the growth of which is much higher than that in the absence ofthese materials (see EP-A-0 549 443): these are referred to as hybridcultures.

[0006] However, these known systems have a number of drawbacks. Thus, inthe method described above, the production of biological sludge is tiedto the normal growth metabolism of the bacteria decontaminating thewater. Furthermore, the growth medium materials used are held in placein the reaction chamber either by a retention screen (that lets waterthrough, but not the support material) or by means of a specificseparation system. The major drawback of screens is their clogging.

[0007] Starting with these known systems, the objective of the presentinvention was to solve the following two technical problems:

[0008] prevent clogging of the retention screens positioned at theoutlet for the treated water;

[0009] reduce the amount of sludge produced, compared with the amount ofsludge produced by conventional methods carrying out the same biologicalpurification.

[0010] These technical problems are solved by a method for thebiological purification of wastewater in hybrid cultures employingmicroorganisms, at least some of which are fixed to solid supportelements, characterized in that said support elements are set in motionso as to generate turbulence in the reaction medium, the intensity ofwhich turbulence is such that it reduces the production of biologicalsludge, the materials constituting said microorganism support elementsbeing subjected to an abrasion action and to a cleaning action whilestill being retained in said reaction medium, said materials having asurface texture that includes regions protected from the abrasion,allowing the growth of a biomass providing the biological activity, andabrasive regions.

[0011] The desirable level of turbulence so as to obtain the bestresults in implementing the method according to the invention, asdefined above, may be expressed by the energy that is supplied by theaeration and/or stirring means. Preferably, this energy is between 1 and200 watts per cubic meter of reactor and preferably between 2 and 50watts per cubic meter of reactor. Such energy levels per cubic meter maybe economically viable on account of the compact nature of the reactorsemployed in the method according to the invention that are definedbelow.

[0012] According to a preferred way of implementing the method definedabove, the microorganism support material has one dimension, along anyaxis, that is between 2 and 50 mm.

[0013] As mentioned above, the microorganism support material has asurface texture such that the surface has regions protected fromabrasion, allowing the growth of a biomass for providing the biologicalactivity, and abrasive regions making it possible, in the presence of asufficient level of turbulence (as defined above), to exert friction onthe external surfaces of the other particles that are present in thereaction medium.

[0014] The subject of the present invention is also a biological reactorfor implementing the method defined above, this reactor beingcharacterized in that it includes microorganism support retention means,these means being positioned upstream of the means for removing theliquid effluent leaving, after treatment, said reactor, these retentionmeans comprising:

[0015] a screen inclined to the vertical at an angle of between 0 and30° approximately and the separation of the bars of which is determinedso that it lets the water through but not the microorganism supportparticles;

[0016] an air injection rail positioned at the base of said screen andoperating continuously or intermittently so as to flush the screen; and

[0017] a deflector panel parallel to said screen and located upstream ofthe latter.

[0018] In the foregoing, the term “upstream” is understood to mean withrespect to the direction of effluent flow from its entry into thereactor to its discharge therefrom.

[0019] Thus by virtue of the invention, the feature consisting insetting the microorganism support particles in motion, for example byinjecting a gas or by mechanical stirring or else by a combination ofthese two means, combined with the feature whereby the constituentmaterial of the microorganism support particles is retained in thereaction medium, while subjecting said material to an abrasion actionand to a cleaning action, makes it possible, on the one hand, to reduceclogging of the screens retaining the support material and, on the otherhand, to reduce the amount of biological purification sludge normallygenerated compared with a method producing the same purification, thisreduction being around 2 to 50%.

[0020] This is because, since the biological reactor in which the methodaccording to the invention is employed includes an inclined screenprovided with a deflector and with an air injection rail that purges thesurface of the screen, less rapid clogging of the screen is ensured thanthat observed in the reactor vessels according to the prior art. It hasbeen observed that the flow of support materials close to the screen,with an increased velocity because of the presence of the deflector,helps to detach the solid materials liable to be deposited on saidscreen, thus making it possible to reduce the rate of clogging.

[0021] It has also been observed, surprisingly, that a certainturbulence intensity in the reaction medium allows the production ofbiological sludge to be reduced. This phenomenon may be explained by thefact that the turbulence within the medium generates friction such thatthe microorganisms fixed in the form of a biofilm adopt a particularmetabolism. This is because a very high abrasion intensity means thatcertain microorganisms must synthesize substances for increasing themechanical integrity of the biofilm. When the intensity of the abrasionis high enough so that most of the microorganisms adopt this particularform of metabolism, the growth yield (which is generally defined asbeing the amount of cells produced relative to the amount of pollutingmaterial degraded) decreases considerably. This results in a markeddecrease in the amount of sludge produced compared with operation in theabsence of turbulence.

[0022] According to the present invention, the microorganism supportmaterial must have a large surface compared with the volume that itoccupies and, preferably, part of this surface must be protected fromthe turbulence and from collisions, as was explained above. Thus,according to the invention, the surface area of the support material isgreater than 100 m² per cubic meter of material and abrasiveexcrescences are provided on the external surface of said material.Thanks to the latter feature, internal regions are defined that will beable to be colonized by microorganisms in an amount sufficient toachieve the desired biological purification. The abrasive externalsurface may be colonized by microorganisms in the form of a biofilm, butthe intensity of the stirring and of the turbulence will be such thatthis biofilm will be in perpetual reconstitution, thereby directing themetabolism of some of the microorganisms that carry out the purificationtoward a particular form of metabolism and thus limiting the productionof biological sludge.

[0023] According to the invention, the microorganism support elementspreferably have one dimension between 2 mm and 50 mm and the constituentmaterial of said support elements is a plastic obtained, for example,from recycled material, for example polyethylene. Examples ofmicroorganism support particles that can be employed in the methodaccording to the present invention will be described below in greaterdetail.

[0024] The method according to the present invention may be employed inaerobic, anaerobic or anoxic biological treatment modes or in treatmentsystems operating in a combination of these three modes.

[0025] In its application to aerobic purification, the method accordingto the invention is characterized in that the microorganism supportparticles are set in motion by injecting air or an inert gas to whichoxygen has been added, the amount of said gas being determined so as, onthe one hand, to ensure biological purification and, on the other hand,to obtain the necessary turbulence intensity.

[0026] In the case of an application to anaerobic purification or anoxicpurification, the microorganism support elements are set in motion bythe fermentation gas or by a mechanical stirring system.

[0027] In its application to a combined carbon/nitrogen treatmentinvolving two steps, an anoxic step and an aerobic step, with recyclingof the mixed sludge from the aerobic step to the anoxic step, the methodaccording to the invention may be carried out in one or both of saidsteps, preferably in the aerobic step so as to immobilize themicroorganisms that oxidize the ammoniacal nitrogen. It is also possibleto carry out, in the same tank, the anoxic and aerobic steps, the tankthen being aerated intermittently and the stirring during the anoxicphase being carried out by another, especially mechanical, means.

[0028] Further features and advantages of the present invention willbecome apparent from the description given below, with reference to theappended drawings that illustrate an example of its implementation thatis devoid of any limiting character.

[0029] So as to bring out the advantage afforded by the invention asregards reducing the production of sludge, an experimental apparatusdescribed below was used, the results from which will be commented uponlater. The means for retaining the microorganism support materialsemployed in the reactor according to the invention will be describedlater.

[0030] In the figures:

[0031]FIG. 1 is a diagram showing the experimental apparatus used fordemonstrating the reduction in sludge production thanks to theinvention;

[0032]FIGS. 2a to 2 c are curves that demonstrate the results providedby the invention as regards elimination of the COD;

[0033]FIGS. 3a and 3 b are curves showing the cumulative amount ofsludge produced as a function of the cumulative amount of COD eliminatedin each of the two experimental reactor lines used (FIG. 1) and for twodifferent sludge ages;

[0034]FIG. 4 is a schematic view showing the retention means employed inthe reactor according to the invention;

[0035]FIG. 5 is a view, on a larger scale, of a detail of FIG. 4; and

[0036]FIGS. 6, 7a, 7 b and 8 show, schematically, examples ofmicroorganism support materials that can be used in the method accordingto the invention.

[0037] As mentioned above, in order to demonstrate the reduction inbiological sludge production provided by the method according to theinvention, two strictly identical activated-sludge reactor lines wereproduced, each reactor being fed with the same wastewater and operatingunder the same operating conditions. One line constituted the control(it is denoted hereafter by “Control line”) containing no floatingbiomass support material, the other line (called hereafter the “testline”) containing a floating growth support material for the biomass,according to the invention.

[0038]FIG. 1 therefore shows each of the experimental lines. Each linecomprises a biological reactor 8, a settling tank 10, a pH/temperatureprobe 3 and an oxygen probe 2. The reactor 8 is fed via a pump 5 from astorage tank 4 for municipal wastewater that has undergone primarysettling. Discharge from the reactor takes place via an overflow from aliquid/solid separator 9, to the settling tank 10. The decanted waterleaves the plant while some of the sludge is recycled back into thebiological reactor 8 by means of a recirculation pump 6. The excesssludge is removed by means of a purge 11. Each line includes a computer1 for analyzing the results obtained. The biological reactor 8 isstirred by a mechanical stirrer 7 and by aeration, when the latter is inoperation.

[0039] As regards the biomass support material, the reader may refer tothe end of the present description, which gives a few nonlimitingexamples thereof.

[0040] The Test line operates according to the principle describedabove.

[0041] Table I below indicates the principal characteristics of thesetwo reactor lines. TABLE I Principal parameters of the lines ValuesVolume of the reactor (8) 22 liters Volume of the settling  2 literstank (10) Plastic support particles (Test line), made of polyethylene:density 935 kg/m² mean diameter 3 mm geometry irregular particles Volumefill factor 20% (Test line) Mechanical stirrer (7): 2 marine propellersdiameter 10 cm

[0042] Table II below gives the operating conditions for the Control andTest lines. TABLE II Nature of the wastewater Domestic wastewater, afterto be treated primary settling, stored at 4° C. and replenished everythree days. Easily assimilatable carbon source supplement (acetate,ethanol, propionate, starch) supplied during the anoxic phase Municipalwastewater (MWW): COD 350-500 mg/l COD/BOD5 1.5 SS 100-150 mg/l NTK 60-90 mg/l N-NH₄  50-75 mg/l Supplement: COD Approximately equivalentto the COD of the MWW (therefore synthetic COD = 50% of the total feedCOD). It is supplied in the anoxic phase to both lines. Volume loadapplied 1 kg COD/m³ · d Mass load applied* Varied between 0.5 and 1 kgcos/kg VSS.d Sludge age Varied between 3 and 8 days Controlledtemperature 16° C. ± 1° C. Aerated phase/nonaerated phase alternation:duration of the 45 min/45 min phases aeration control Dissolved oxygen >3 mg/l

[0043] Control line: the biomass in equilibrium is smaller for the Testline.

[0044] The two lines operated with a continuous feed of wastewater andwith a flow rate making it possible to obtain a mean applied load of 1kg of COD per cubic meter of reactor per day.

[0045] The biological reactor 8 operated both with aeration and stirringand with only stirring. This mode of operation made it possible toalternate the aerobic phases, ensuring nitrification of the speciescontaining ammonia (denoted by N—NH₄ in Table II) present in thewastewater (i.e. their conversion into oxidized species such as nitritesor nitrates), and the anoxic phases for denitrification (i.e. theconversion of the oxidized species into molecular nitrogen).

[0046] This mode of operation allowed all of the steps of eliminatingthe nitrogen contamination to be carried out in the same reactor.

[0047] During the aerobic phases, the dissolved oxygen concentration wasmaintained at above 3 mg/l. During the anoxic phases, a certain amountof organic carbon, taken from an external carbon source 12, was added tothe reactor 8 so as to reduce the time needed for the denitrificationstep.

[0048] During the experiment, the sludge age (that is to say the ratioof the total amount of biological sludge contained in the experimentaldevice, the settling tank included, to the amount of biological sludgeextracted) varied between 3 and 8 days. This parameter was adjusted bythe rate of purge 11 of the biological sludge.

[0049] The measurements taken relate to all of the parameters that makeit possible to characterize the budgets of the contamination enteringand leaving the apparatus: total and soluble chemical oxygen demand,ammoniacal nitrogen N—NH₄, nitrites and nitrates. The amount of sludgeis quantified on the basis of the suspended solids (SS) and of volatilesuspended solids (VSS).

[0050] The sludge production is calculated as being the sum of sludgeextracted by the purge, the amount of sludge leaving in the decantedeffluent and the accumulation of sludge in the biological reactor (infree form or in fixed form).

[0051] An apparent biomass yield Y_(obs), that is to say the ratio ofthe amount of sludge produced to the amount of COD removed by thesystem, was also calculated.

[0052] The results obtained are illustrated by FIGS. 2a and 2 c, whichshow the variation in the load removed as a function of the loadapplied. These figures show that there are no substantial differences,as regards the amounts of COD removed, between the Control line and theTest line.

[0053] Referring now to FIGS. 3a and 3 b, these show the cumulativeamount of sludge produced as a function of the cumulative amount of CODremoved, in each of the two lines (the Test line and the Control line)and for two different sludge ages. The curves illustrated by thesefigures demonstrate that the amount of sludge produced, expressed on thebasis of the amount of volatile suspended solids, is lower in the Testline than in the Control line. The slope of each of the curvesrepresents the current biomass yield, allowing the results thus obtainedto be compared. It will be seen that, for a sludge age of 8 days, thebiomass yield obtained in the Control line is 0.4 kg VSS/kg COD, whereasit is 0.24 kg VSS/kg COD in the Test line. The observed reduction issubstantial (around 40%). With a sludge age of three days, the apparentyield is 0.44 for the Control line and 0.32 for the Test line, i.e. areduction of 27%. It will be recalled that the only difference betweenthe two reactor lines is the presence of growth medium support materialin the Test line, with a volume fill factor of 20%.

[0054] Although at the present stage of the experiments the surprisingresults obtained by implementing the method of the invention cannot beformulated into a complete theory, it is possible however to provideseveral explanations.

[0055] Firstly, it should be noted that the observed differences betweenthe results obtained on the Control and Test lines are clearly due to adifferent metabolism of the microorganisms when they are fixed to theirsupport and set in motion by mechanical stirring and/or aeration:

[0056] it is clear that the fixed bacteria has a residence time in thereactor that is much longer than the free bacteria. Consequently, thecell mortality is higher, resulting in a lower production of sludge.However, this factor cannot by itself justify a 27 to 40% lower sludgeproduction as mentioned above;

[0057] the fixed microorganisms and the bacterial flock particlespresent in the culture medium of the biological reactor of the Test lineundergo mechanical work due to the stirring and to the abrasion betweenthe particulate materials, because of collisions between the particles.It is known that the fixed microorganisms are structured as a biofilmand the cohesion of this biofilm is provided by exopolymers synthesizedby the bacteria. Large mechanical stresses contribute to the destructionof this structure; Maintaining a biological activity on the materialtherefore requires a continuous synthesis of exopolymers by thebacteria. As a result, the synthesis of these polymers becomes a moreimportant metabolic pathway than the production of sludge. Since theseexopolymers are either partially biodegradable, or soluble, they areinvolved in the abrasion mechanism in the liquid effluent.

[0058] A larger reduction in sludge for a greater sludge age, as FIGS.3a and 3 b show, may corroborate this second hypothesis insofar as theduration of the mechanical stress exerted on the biomass is longer.

[0059] It was seen above that the use of support materials for thegrowth of the microorganisms required particular means for retainingthese materials in the biological reactor chamber. An embodiment of theretaining means thus employed will now be illustrated with reference toFIGS. 4 and 5.

[0060] These figures show that this retention device, which is placed infront of the chute 17 at the outlet of the reactor 13 for the treatedeffluent, essentially comprises a screen 15 inclined to the vertical ofan angle α of preferably between 0 and 30°. The spacing of the bars ofthe screen is determined so as to let the water through, but not themicroorganism support particles. The spacing of these bars is thereforeless than the smallest dimension of the support particles used forimmobilizing the microorganisms. A deflector panel 16 is placed parallelto the screen, upstream of the latter in the reactor 13. Provided at thebase of the screen 15 is an air injection rail 14 for flushing thescreen continuously or intermittently. The combined effect of thisdeflector panel 16 and of the flushing thus produced allows theascending liquid flow to be channeled by an “air lift” effect that alsoentrains the particles of microorganism growth support materials 18(FIG. 5). The flow thus created has two advantages:

[0061] firstly, the particles of support material help to clean thescreen 15; and

[0062] secondly, the high mechanical stresses exerted on the surface ofthe particles of support material in this region improve the sludgereduction effect observed experimentally and as mentioned above.

[0063] The treated liquid effluent discharged from the biologicalreactor, passing through the screen 15, is then removed by overflow bymeans of a spillway to the chute 17.

[0064] As regards the microorganism support elements, according to thepresent invention it is possible to use any existing material availablecommercially or able to be manufactured in accordance with theabovementioned characteristics. This material must therefore have thefollowing characteristics:

[0065] one dimension, taken along any axis, of between 2 and 50 mm;

[0066] a particular surface texture, namely the presence of regionsprotected from abrasion (that allow the growth of a biomass, providingthe biological activity) and abrasive regions that make it possible, inthe presence of a high enough level of turbulence as defined above, toexert friction on the external surface of the other particles present inthe reaction medium.

[0067] Thus, by taking into consideration the above-mentionedcharacteristics, a person skilled in the art will be able to select thetypes of materials suitable for the operation that has to be carriedout. A few nonlimiting examples of materials that can thus be used aregiven below.

EXAMPLE 1 Particulate Material.

[0068] Microorganisms support elements are formed from granularparticles that can be obtained from the recycling of plastics, asdescribed, for example in FR-A-2 612 085. FIG. 6 of the appendeddrawings illustrates an example of such particles that are in the formof granules having a very irregular shape, with recesses 20 protectedfrom abrasion and protruding parts 19 that promote abrasion. The size ofthese granules is between 2 and 5 mm and their developed surface areamay be between 5000 and 20 000 m²/m³.

EXAMPLE 2 Extruded Plastic.

[0069] In this case, the microorganism support elements are formed fromextruded and cut plastic materials. FIGS. 7a and 7 b of the appendeddrawings show end and side views, respectively, of an illustrativeexample of such an element. This element is cylindrical in shape and hasribs 21, 22 provided on its external and internal surfaces respectively.The external ribs 21 allow the abrasion action to take place while theinternal ribs 22 increase the surface area available for colonization ofthe biomass. The size of these support elements may be between 5 and 25mm and their total developed surface area may be between 100 and 1500m²/m³.

EXAMPLE 3 Compression-Molded Or Injection-Molded Plastic.

[0070] It is known that there are on the market many types of packingelements for columns having the required characteristics for takingadvantage of the present invention. FIG. 8 of the appended drawingsshows, in perspective, three illustrative examples of elements of thistype. They are generally referred to as rings. Their size may be between10 and 50 mm and their developed surface area may be between 100 and1000 m²/m³. In the rings illustrated in FIG. 8, the abrasive surfacesmay be the edges of the cylinders 24 and the recessed parts 23.

[0071] It will be noted that, with this type of material, which ischaracterized in particular by a larger size than the previous ones, theabrasion is also effected by the liquid flow through the internalregions. The rings include internal ribs 25 for colonization by themicroorganisms.

[0072] Of course, it remains to be stated that the present invention isnot limited to the illustrative examples described and shown above,rather it encompasses all variants thereof.

1. A method for the biological purification of wastewater in hybridcultures employing microorganisms, at least some of which are fixed tosolid support elements, characterized in that said support elements areset in motion so as to generate turbulence in the reaction medium, theintensity of which turbulence is such that it reduces the production ofbiological sludge, the materials constituting said microorganism supportelements being subjected to an abrasion action and to a cleaning actionwhile still being retained in said reaction medium, these materialshaving a surface texture that includes regions protected from theabrasion, allowing the growth of a biomass providing the biologicalactivity, and abrasive regions.
 2. The method as claimed in claim 1,characterized in that the intensity of the turbulence generated in thereaction medium, defined by the energy supplied by the means foraerating and/or stirring said medium, is between 1 and 200 watts percubic meter of reactor and preferably between 2 and 50 watts per m³ ofreactor.
 3. The method as claimed in either of claims 1 and 2,characterized in that the reduction in biological purification sludgeproduction is around 2 to 50% relative to the sludge production obtainedin conventional processes achieving the same biological purification. 4.The method as claimed in claim 1, characterized in that the surface areaof the constituent material of said solid microorganism supportparticles is greater than 100 m² per cubic meter of material.
 5. Themethod as claimed in either of claims 1 and 4, characterized in that themicroorganism support elements have a size of between 2 mm and 50 mm. 6.The method as claimed in any one of claims 1, 4 and 5, characterized inthat the constituent material of said microorganism support elements isa plastic.
 7. The method as claimed in any one of claims 1 and 4 to 6,characterized in that the constituent material of said microorganismsupport elements is a granular material having recessed parts (20)protected from abrasion and projecting parts (19) that promote abrasion.8. The method as claimed in any one of claims 1 and 4 to 6,characterized in that the constituent material of said microorganismsupport elements is formed from extruded plastic elements cut inparticular in the form of a cylinder and provided with external ribs(21) that promote abrasion and internal ribs (22) for colonization ofthe biomass.
 9. The method as claimed in any one of claims 1 and 4 to 6,characterized in that the constituent material of said microorganismsupport elements is formed from injection-molded or compression-moldedplastic packing elements, in particular having the shape of cylindricalrings whose edges (24) and recessed parts (23) promote abrasion, theserings having internal ribs (25) for colonization by the biomass.
 10. Themethod as claimed in any one of the preceding claims, applied to aerobicpurification, characterized in that the microorganism support particlesare set in motion by injecting air or an inert gas to which oxygen hasbeen added, the amount of said gas being determined so as, on the onehand, to ensure biological purification and, on the other hand, toobtain the necessary turbulence intensity.
 11. The method as claimed inany one of claims 1 to 9, applied to anaerobic purification or to anoxicpurification, characterized in that the microorganism support elementsare set in motion by injecting the fermentation gas.
 12. The method asclaimed in any one of claims 1 to 9 applied to anaerobic purification orto anoxic purification, characterized in that the microorganism supportelements are set in motion by mechanically stirring the reaction medium.13. The method as claimed in any one of claims 1 to 9 applied tocombined carbon/nitrogen treatment, in which it is carried out in twosteps, an anoxic step and an aerobic step, with recycling of the mixedsludges from the aerobic step to the anoxic step, characterized in thatit is applied to at least one of said steps.
 14. The method as claimedin claim 13, characterized in that it is applied to the aerobic step soas to immobilize the microorganisms that oxidize ammoniacal nitrogen.15. The method as claimed in claim 13, characterized in that the anoxicand aerobic steps are carried out in the same tank, the latter beingaerated intermittently and the stirring during the anoxic phase beingprovided by another means, especially such as mechanical stirring.
 16. Abiological reactor for implementing the method as claimed in any one ofthe preceding claims, characterized in that it includes microorganismsupport retention means positioned upstream of the means for removingthe liquid effluent leaving the reactor (13) and comprising: a screen(15) inclined to the vertical at an angle (α) of between 0 and 30° andthe separation of the bars of which is determined so that it lets thewater through but not the microorganism support particles; an airinjection rail (14) positioned at the base of said screen and operatingcontinuously or intermittently so as to flush the screen; and adeflector panel (16) parallel to said screen (15) and located upstreamof the latter.